1. Field of the Invention
The present invention relates to a front-end circuit provided between a high frequency circuit such as a transmitting and receiving circuit and an antenna, and a communication terminal apparatus equipped with the front-end circuit.
2. Description of the Related Art
Between a high frequency circuit and an antenna, generally, an impedance matching circuit that performs impedance matching between the high frequency circuit and the antenna is provided. For example, since compact communication terminal apparatuses such as mobile phones have a relatively small antenna size for a communication frequency band, the impedance of an antenna tends to be smaller than the characteristic impedance of a feeding circuit. Therefore, a large impedance conversion ratio is required. As a circuit that performs constant impedance conversion, for example, Japanese Patent Laid-Open Publication No. 2004-072445 discloses a transformer-type matching circuit.
In recent years, communication terminal apparatuses, such as mobile phones, may require compatibility with communication systems, such as a GSM (registered trademark) (Global System for Mobile communication), DCS (Digital Communication System), PCS (Personal Communication Services), and UMTS (Universal Mobile Telecommunications System), as well as a GPS (Global Positioning System), a wireless LAN, Bluetooth (registered trademark), and the like. Thus, antenna devices for such communication terminal apparatuses are required to cover a wide frequency band from 800 MHz to 2.4 GHz.
On the other hand, since antennas generally have a frequency characteristic in the impedance, it is difficult to obtain favorable matching in a wide band by using such a transformer-type matching circuit disclosed in Japanese Patent Laid-Open Publication No. 2004-072445, that performs constant impedance conversion.
FIG. 1 illustrates an example of the frequency characteristic of the transformer-type impedance conversion circuit as disclosed in Japanese Patent Laid-Open Publication No. 2004-072445. FIG. 1A illustrates return loss as viewed from a feeding port of an impedance conversion circuit to the antenna side in a frequency range from 700 MHz to 2.3 GHz, and FIG. 1B illustrates an impedance trace on a Smith chart.
This example illustrates the return loss is minimum at about 1.9 GHz, that is, the radiation efficiency from the antenna is maximum, and in other frequencies, for example, in the 800 MHz band, the return loss is large and the radiation efficiency is extremely poor. Therefore, the impedance conversion circuit disclosed in Japanese Patent Laid-Open Publication No. 2004-072445 cannot be used for the above described wideband antenna device.
In addition, the antenna devices for a wide frequency band typically have a wideband matching circuit including an LC parallel resonant circuit or an LC series resonant circuit. However, in the LC circuits, some insertion loss is generated, which generates a signal energy loss.
Depending on arrangement positions of a ground plate, a casing, other closely disposed components (in particular, high frequency components such as an antenna and a strip line), the characteristic of an impedance conversion circuit may change.